Pre-press color proofing is a procedure used by the printing industry to create representative images of printed material. This procedure avoids the high cost and time required to produce printing plates and set-up a high-speed, high-volume printing press to produce a single intended image for proofing prior to a production run of the intended image. In the absence of pre-press proofing, a production run may require several corrections to the intended image to satisfy customer requirements, and each of the intended images would require a new set of printing plates. By utilizing pre-press color proofing, time and money are saved.
A laser thermal printer having halftone color proofing capabilities is disclosed in commonly assigned U.S. Pat. No. 5,268,708 (Harshbarger, et al.). The U.S. Pat. No. 5,268,708 device is capable of forming an image on a sheet of thermal print media by transferring dye from dye donor material to thermal print media. This is achieved by applying thermal energy to the dye donor material to form an image on the thermal print media. The apparatus disclosed comprises a material supply assembly; a lathe bed scanning subsystem, which includes a lathe bed scanning frame, a translation drive, a translation stage member, and a laser printhead; a rotatable vacuum imaging drum; and exit transports for the thermal print media and dye donor material.
The U.S. Pat. No. 5,268,708 apparatus meters a length of the thermal print media in roll form from a material supply assembly. The thermal print media is measured and cut into sheets of the required length, transported to the vacuum imaging drum, and wrapped around and secured to the vacuum imaging drum. Donor roll material is metered out of the material supply assembly, measured, and cut into sheets of the required length. A sheet of dye donor material is transported to and wrapped around the vacuum imaging drum, and superposed in registration with the thermal print media. The scanning subsystem traverses the printhead axially along the rotating vacuum imaging drum to produce the image on the thermal print media. The image is written in a single swath, traced out in a continuous spiral, concentric with the imaging drum, as the printhead is moved parallel to the drum axis.
The U.S. Pat. No. 5,268,708 apparatus simulates the printing process by imaging the dye donor material at a constant exposure. The dye donor is used to mark or not mark the thermal print media similar to the printing process, which either transfers or does not transfer ink. The apparatus allows the exposure to each dye donor material to be varied over a limited range to allow the customer to match the density of the of the dye deposited on the thermal print media with the density of the ink which will be used to print the image on a press.
Color proofers create halftone bitmaps of cyan, magenta, yellow, and black color planes using a raster image processor (RIP). Customer artwork is composed into pages using software such as Quark Express™ or Adobe InDesign™. These pages may consist of color images, black and white images, artwork, linework, and text. Images may be continuous tone, multilevel, or binary. The pages may also contain PDF or PostScript codes. The RIP processes the input pages and creates halftone bitmap files for each color plane at the writing resolution of the printer. The RIP converts multilevel input, such as the pixels in a continues tone image, into halftone dots of the appropriate size.
Although the printer disclosed in U.S. Pat. No. 5,268,708 performs well, there is a long-felt need to be able to use colors other than the standard cyan, magenta, yellow, and black (CMYK) in pre-press color proofing. The CMYK colors are often referred to as process colors. In the printing industry additional colors, other than cyan, magenta, and yellow, are used depending upon the graphic designers intentions for the printed work. A “key” color may be added to highlight a particular component of the artwork. For screened continuous tone images this key color is typically chosen to be black. The image is modified to use black to adjust the intensity level within the image instead of using cyan, magenta, and yellow together. This is called under color removal. For some work, the customer may choose to use another color, for example, brown, as the key color. This may be appropriate, for example, on a cereal box or in an image with a tan subject. To print the job the printer uses cyan, magenta, yellow and the key color. To save money one or more of the colors may be eliminated. For artwork the printing industry may print the job with the exact inks used by the artist. In these cases the printer may be printing red, blue, or some combination of colors which may or may not include CMYK.
In many cases the color of the subject may not be successfully reproduced using the standard CMYK colorants. In this case an additional color printing plate may be created to be printed with an ink which is a close match to the desired color of the subject. This additional color is imaged with the CMYK layers and is called a “bump” plate. It is important to note that one or more of the process colors may be eliminated or replaced with the bump color. For instance if a red color is used to bump the color of a red car, then the black or cyan process color may be replaced with the red bump color.
In existing pre-press systems, additional donor colors would be needed to accomplish this. For example, commercial systems such as Polaroid Graphics Imaging Polaproof, Dupont Digital Halftone Proofing System, and Imation Matchprint Laser Proof Technology, have all advertised the availability of additional donor colorants to create digital halftone proofs with special colors. This solution, however, requires the manufacturer to produce additional dye donor sheet in special color, often in small volume. Small production runs like this, for one color, are expensive.
Another problem arises when plates in the printing press are out of register. In that case colors are imaged slightly wider and overlapped so that a white space error does not occur. The printing industry hides this defect by increasing the line width of a color such that errors in color placement are hidden behind the darker color. This technique is called “trapping.” It is important to be able to see the trapping on each of the printed color planes in the halftone proof. The capability to show trapping is not readily available in state of the art pre-press color proofers without use of a special color dye donor sheet.
Printing presses traditionally uses a halftone screen to generate a tone scale. The printing process is only capable of delivering or not delivering ink, which is usually opaque. This is the binary printing process. To generate a light tint, small dots of ink are used. To generate a darker tint the ink dots are enlarged which touch and fill the space between dots. The halftone proofer disclosed in U.S. Pat. No. 5,268,708 images CMYK colorants at a high resolution. For example, a Kodak Approval XP system produces images at either 2400 dpi or 2540 dpi using a software raster image processor (RIP) to generate a bitmap which determines when the lasers within the printer mark the CMYK films. The colors are arranged in a grid and the pitch, dot to dot, is called the screen ruling in dots per inch. The angle of the grid is called the screen angle. Each color is printed at a different screen angle to hide the beating, or aliasing, caused by the alignment and accuracy of the color screens to each other. For optimum conditions the cyan, magenta, and black screens are each separated by 30 degrees. The fourth color, yellow, is then placed at an angle half way between the angle of two of the other colors. Each color screen is separated out as a separate bitmap plane with a grid of pixels at the writing resolution of the digital halftone proofer. The software RIP determines the positions in the grid when the laser needs to be energized to print the halftone dot.
It is common practice in color proofing to represent special color planes, i.e. planes containing colors other than the processes colors, by replacing solid color areas with halftone patterns of the process colors as described for example in U.S. Pat. No. 5,309,246 (Barry et al.). It is usually necessary to attach additional instructions with these proofs to inform customers and printers that a substitution has been made. It is highly desirable for halftone color proofing systems to reproduce the special color planes with colors that more closely represent the final print job. In the case of laser thermal material transfer proofing systems it is well known that this can be accomplished by using individualized donors having the unique color required for the special color plane, however, this process adds additional expense as described above.
U.S. Pat. No. 5,309,246 generates special colors using separate screens of the primary components of the special color. These screens consists of halftone dots of different sizes in each of the primary colors to compose a “recipe” color and simulate the special color required in the proof. If the halftone screens of the recipe color are designed to run at a constant density of the primary color then they may be combined and run in one pass. Trapping would not be shown if they run in a single pass. The special color is obtained by modulated density using halftone dot size so that the halftone dot size of the written special color will not match the press. The advantage of U.S. Pat. No. 5,309,246 is that the halftone special color is a good approximation of the special color. The disadvantage of U.S. Pat. No. 5,309,246 is that an additional exposure pass is required for each component of the special color.
U.S. Pat. No. 6,060,208 (Wang) simulates the special color by screening the special color, then modifying the screened bitmap by eliminating pixels so that when the modified bitmap is imaged with the printer's primary color less density is printed and the resulting proof looks like it has been imaged with an additional special colorant. The method in U.S. Pat. No. 6,060,208 also produces a halftone image of the special color. However, the method in U.S. Pat. No. 6,060,208 has noticeable holes within the special color halftone dots on the proof. For cases where a small amount of pixels are subtracted or a few number of pixels are added, the method in U.S. Pat. No. 6,060,208 is similar to U.S. Pat. No. 5,309,246. With the method in U.S. Pat. No. 6,060,208 the special color bitmap may be combined with the primary color bitmap so that the primary color is imaged in a single pass. However, the exposure for this pass is a constant, and the density transferred per micro-pixel is also a constant. If the printer resolution is high enough to see each individual micro-pixel in the image then the method in U.S. Pat. No. 6,060,208 is visible in the halftone proof. If the printer resolution is lower than the writing resolution then the missing or additional micro-pixels are blurred in the image and the method in U.S. Pat. No. 6,060,208 is modulating the density in the halftone proof identical to U.S. Pat. No. 5,309,246. When a few micro-pixels are added or subtracted from the primary color then the method in U.S. Pat. No. 6,060,208 produces pleasing results. However, when a small amount of primary color is required then using a few micro-pixels at a fixed density does a poor job creating a halftone screen of the special color. This is described by U.S. Pat. No. 5,309,246 as adding a small halftone dot of one color to a larger halftone dot of a second color to achieve a third color.
Commonly-assigned U.S. patent application Ser. No. 09/535,671, filed Mar. 23, 2000 describes a method of imaging a special color using multiple primary colors at unique exposures to mix different amounts of primary colors on the halftone proof. U.S. patent application Ser. No. 09/535,671 uses a single binary bitmap for each of the colors in the special color. The disadvantage of U.S. patent application Ser. No. 09/535,671 is that an additional exposure pass is required for imaging each additional colorant within the special color.
Color proofers may use both laser power and drum speed to adjust the exposure for each colorant. A drum speed increment of 25 RPM allows running close to the maximum laser power most of the time thereby increasing print throughput. Color proofers image one bitmap at one exposure per pass. The high writing resolution and the small spot size, approximately 25 um, are used to simulate center weighted halftone dots and text that are normally imaged on a printing press. Printing special colors using multiple exposures of the primary colors requires additional exposures passes. These extra passes take more time and lower the throughput of the digital halftone proofer.